Add support for registering existing interfaces

This adds support for native and slcan interfaces which are already
configured up to be registered in the CAN bus HAL.

Bug: 142655647
Test: manual
Change-Id: Ifd129db14dbf473bb627ebc9b9d13f5cb945b611
diff --git a/automotive/can/1.0/default/CanBusNative.cpp b/automotive/can/1.0/default/CanBusNative.cpp
index 88f9175..047b090 100644
--- a/automotive/can/1.0/default/CanBusNative.cpp
+++ b/automotive/can/1.0/default/CanBusNative.cpp
@@ -31,6 +31,11 @@
         return ICanController::Result::BAD_ADDRESS;
     }
 
+    if (mBaudrate == 0) {
+        // interface is already up and we just want to register it
+        return ICanController::Result::OK;
+    }
+
     if (!netdevice::down(mIfname)) {
         LOG(ERROR) << "Can't bring " << mIfname << " down (to configure it)";
         return ICanController::Result::UNKNOWN_ERROR;